1. Field of the Invention
The present invention relates to a metal halide lamp having improved characteristics of color temperature of lamp light.
2. Related Background Art
Metal halide lamps are used nowadays often in its specific applications. For example, if light source color of the setting sun, light source color of the rising sun, cool white of the sun, and daylight of the blue sky are intended to be radiated for performance, lamps having color temperatures of 2000 to 3000 K, 3000 to 4000 K, 4000 to 5000 K, and 6000 to 7000 K respectively are used.
A metal halide lamp having an arc tube filled in which is a combination of metal iodides such as dysprosium (Dy) iodide and thallium (Tl) iodide or a combination of metal iodides such as dysprosium (Dy) iodide and neodymium (Nd) iodide, is characterized in its high luminous efficacy and high color rendering property. Application fields such as indoor illumination by such metal halide lamps are expanding and demands for such lamps are increasing.
A color temperature of a metal halide lamp is determined by a kind of a fill in the arc tube. Therefore, each metal halide lamp having its specific color temperature is not suitable for the performance of a light source color of a different color temperature.
A technique is known which converts a color temperature of a light source by forming an optical interference coating on the surface of a light transmissive substrate enclosing the light source. With this technique, a lamp having a desired color temperature can be manufactured by using an optical interference coating having predetermined spectral transmittance characteristics. Such proposals have been made conventionally to lower a color temperature of a metal halide lamp having a high color temperature to a desired value by using an optical interference coating (usually a multilayer coating). A spectral distribution of light transmitted through a multilayer coating formed on the surface of an arc tube of a metal halide lamp or on the surface of a light transmissive cylindrical tube enclosing the arc tube, changes with the spectral transmittance characteristics of the coating, and the color temperature of the lamp lowers. These proposals have not recognized at all the fact that it is necessary to make the spectral transmittance characteristics have predetermined conditions matching the spectral irradiance of light in the arc tube in order to lower a color temperature by a desired quantity. Therefore, even if these proposals are used in practice, a desired quantity of lowering a color temperature cannot always be ensured. Specifically, first, of layer constitution conditions determining the spectral transmittance characteristics of a multilayer optical interference coating, the condition of a layer thickness (optical thickness) is not positively given. Therefore, depending upon a layer thickness, a quantity of lowering a color temperature becomes insufficient or a color temperature rises so that a desired color temperature cannot be obtained. Moreover, in some cases, a color rendering performance is degraded and a total luminous flux is reduced. Second, the condition of a number of layers is not positively given. Therefore, if the number of layers is small, a quantity of lowering a color temperature is insufficient, whereas if the number of layers is too large, a color temperature lowers excessively below a desired color temperature, and at the same time a color rendering performance of the lamp is degraded and a total luminous flux is reduced.
A low color temperature of about 4000 K or lower is difficult to be realized by a conventional metal halide lamp- Even if it is realized, the color rendering performance is very poor.
To realize a color temperature of 2000 to 3000 K, a high pressure sodium lamp has been used conventionally because it is difficult for a metal halide lamp to realize this temperature range. Of the color rendering performance of a high pressure sodium lamp, Ra is about 85 at the most. Although various improvements have been made, a lamp with Ra of 90 or higher has not been realized as yet.
A color temperature of 3000 to 4000 K has recently been used intentionally for shop illumination, performance illumination, and the like because this temperature range provides calm atmosphere. This temperature range can be realized by conventional metal halide lamps. For example, a metal halide lamp having scandium (Sc)--sodium (Na) based halide as a fill in the arc tube. Ra value of this lamp is about 65 to 70 . Ra of 90 or higher has not been realized even by other kinds of metal halide lamps.
A multilayer optical interference coating may be used for raising a color temperature by setting a proper thickness of each layer of the coating. However, in this case, there are a function of raising a color temperature and a function of lowering a color temperature by the thermal insulation effect of the coating. Because beth the functions cancel out, it is difficult to finely control a rise of a color temperature by the coating. It is not effective therefore for controlling a color temperature by the coating if a color temperature of a lamp in a coating-less state is lower than a target color temperature.